Electric discharge machining makes it possible to cut a workpiece made of a hard metal, or the like, into any shape by means of a soft metal electrode made of copper, brass, or the like. In particular, electric discharge machines that have come into ever wider use in recent years are adapted to achieve highly precise machining control by way of computerized numerical control, or CNC, based on the processing of software through use of a minicomputer or microcomputer. Among these machines, a wire-cut electric discharge machine is capable of performing highly precise cutting irrespective of workpiece material and hardness and exhibits greater effectiveness the more complicated the shape into which the workpiece is to be cut.
Furthermore, since water can be used as the machining liquid, an additional advantage is that the machine can operate at night without fear of fire. The demand for such wire-cut electric discharge machines is growing.
FIG. 1 is a view showing the construction of a conventional wire-cut electric discharge machining apparatus. The conventional wire-cut electric discharge machining apparatus will be described on the basis of FIG. 1.
The wire-cut electric discharge machining apparatus is basically composed of a machine unit A, a machining power supply unit B, and a control unit C. The machine unit A comprises a table drive, a wire drive and a machining liquid supply unit. The table drive generally is referred to as an XY table on which a workpiece 60 is placed for imparting the workpiece with movement relative to a wire 80, which is an electrode. The wire drive is for feeding the wire 80 in a uniform manner.
More specificially, the wire 80 is fed from a wire supply bobbin 81 and is taken up on a wire take-up roller 82. Since the wire 80 repeatedly undergoes complex vibration, guides 70a, 70b are provided at predetermined positions relative to the workpiece 60 in order to support the wire.
The machining liquid supply section uses water as the machining liquid. Pure liquid contained in a pure liquid tank 100a is supplied by a pump 20 to upper and lower nozzles 50a, 50b respectively via piping 30 and is jetted as machining liquid from the upper and lower nozzles 50a, 50b into the gap between the wire 80 and workpiece 60. The jetted liquid is recovered as contaminated liquid in a contaminated liquid tank 100b, whence the liquid is drawn up by a pump 21, filtered of sludge, or the like, by a filter 22 and returned to the pure liquid tank 100a following an ion exchange for maintaining the machining liquid at a predetermined conductivity.
The machining power supply unit B supplies discharge energy across the wire 80 and workpiece 60. The control unit C is constituted by a numerical control device (CNC) and is adapted to control the various sections of the machine unit A.
In wire-cut electric discharge machining, the higher the liquid pressure of the machining liquid jetted between the wire 80 and the workpiece 60, the higher the flow velocity, the better the discharge of machining chips produced in the small gap between the wire 80 and the workpiece 60, and the higher the electric discharge efficiency in the direction of machining advance. This enables an increase in machining speed.
As an example if the rate at which water is jetted from both the upper and lower nozzles 50a, 50b respectively is increased by 10 l/min, the machining speed will increase. However, the width of the groove cut in the workpiece will increase greatly when the discharge energy temporarily becomes small in magnitude for some reason during the course of machining and machining is performed at low speed; or the machining energy is reduced and machining speed is slowed down for such purposes as, e.g., preventing breakage of the wire at a corner portion or improving dimensional precision. These conditions will now be described with reference to FIGS. 2 and 3.
FIG. 2 is a view showing an ideal machining example which illustrates the width of a groove cut in a workpiece, and FIG. 3 is a view showing a prior-art machining example which illustrates a workpiece in which the width of a cut groove increases at a corner portion. Thus, ideally, machining is performed in such a manner that the width of the cut groove at linear portions remains unchanged even at corners, as shown in FIG. 2. However, when a corner portion is cut by reducing the discharge energy and lowering machining speed without changing the flow rate of the machining liquid, a problem arises wherein the width of the cut groove increases at the corner portion, as depicted in FIG. 3.
More specifically discharge energy is high when machining is performed at a high speed, and there is a tendency for the wire to be strongly constrained to the center of the cut groove by discharge pressure during machining. Consequently, oscillation of the wire caused by the liquid pressure of the machining liquid has little influence. However, when there is a transition to low-speed machining at a corner portion, or the like, and discharge energy is reduced, there is a weakening of the force ascribable to the discharge pressure that constrains the wire to the center of the cut groove, and the wire is thus caused to oscillate by the liquid pressure of the machining liquid. This enlargement of the cut groove width caused by wire oscillation presents a problem in the prior art.
Though it is possible to diminish the influence of machining liquid pressure on the width of the cut groove by such expedients as increasing wire tension, increasing wire hardness or providing the wire guides closer to the workpiece, there is a practical limit to the amount of wire tension, wire diameter and wire strength in an actual machining operation and it is difficult to bring the wire guides close to the workpiece surface, especially if the workpiece is small in size.